The present invention relates to a compressor arrangement for generating compressed air by a motor-driven compressor unit, downstream of which is positioned at least one cooler unit for cooling the generated compressed air. The compressed air enters into the cooler unit via at least one inlet, flows through a cooling structure around which cooling air flows and which is composed of a plurality of parallel cooling ducts, and leaves the cooler unit via at least one outlet. A bypass means is provided between the inlet and the outlet of the cooler unit in order to prevent freezing of the cooler unit at low ambient temperatures.
The field of application of the present invention extends primarily to oil-free piston compressors which are used for generating compressed air for example in the utility vehicle and rail vehicle industry. In such applications, the compressor arrangement must function over a temperature range from −50° C. to +50° C. ambient temperature. Since the cooler design also provides a maximum cooling power at +50° C., it is possible under special ambient conditions—for example 100% air humidity, 20° C. ambient temperature and 50% activation duration—to observe icing up of the cooling ducts within the cooler unit of the compressor arrangement. During the operation of the compressor unit, said icing can, over a time period of several hours, progress to such an extent that a sufficient delivery of air is no longer possible and the compressor arrangement ultimately fails.
U.S. Pat. No. 6,952,932 B2 discloses a cooler unit for a compressor arrangement in which the above-explained problem is solved in that not the entire quantity of compressed air heated by the compressor unit positioned upstream passes through the cooling structure around which cooling air flows. Rather, a part of the heated compressed air is conducted via a bypass line past the cooling structure directly to the outlet, where said compressed air is mixed with the compressed air cooled by the cooling structure. For the mixing, however, a mixing valve inserted into the bypass line is necessary, which mixing valve is actuated according to an electronic controller in order to mix hot compressed air with cooled compressed air according to the ambient temperature.
A disadvantage of said known technical solution is however that the valve and control technology provided here entails a corresponding level of device-related expenditure. In the event of a failure or malfunction of valve or control electronics as a result, for example, of an interruption in the power supply to the control electronics or wear of a seal in the valve, freezing of the cooling unit could progress unhindered to the point of its destruction. Furthermore, said solution is suitable primarily for preventing freezing of the units positioned downstream of the cooler unit.
U.S. Pat. No. 5,669,363 discloses a further technical solution for bypass means on a cooler unit, which however likewise requires extremely complex valve control—in this case by means of a thermostat valve.
It is therefore the object of the present invention to create a compressor arrangement whose at least one cooler unit is effectively prevented from freezing by means of simply-constructed bypass means.
The object is achieved proceeding from a compressor arrangement as follows.
The invention encompasses the technical teaching that merely a permanently open bypass pipe, without any valves situated in the compressed air flow, is provided as a bypass means between the region of the inlet and the region of the outlet of the cooler unit. Here, the inner cross section of the bypass pipe is matched to the delivery capacity of the compressor unit and to the pressure difference between the inlet and the outlet in such a way that the bypass pipe generates a higher flow resistance than the cooling structure at normal ambient temperatures. Normal ambient temperatures refer primarily to temperatures above the freezing point. At temperatures below the freezing point and with progressive freezing of the cooling structure, the compressed air however flows to an increasing extent via the bypass pipe.
The advantage of the solution according to the invention is in particular that, by means of said specially dimensioned bypass pipe, the compressed air generation can be maintained with a minimum loss of cooling and delivery capacity in the event of freezing of the cooler unit. Since the invention gets by entirely without valves, it can be implemented with a conceivably low amount of technical expenditure. The solution according to the invention has an automatic regulating action based on the dimensioning of the bypass pipe. Here, the bypass pipe is to be designed such that the outlet temperature (measured downstream of the cooler unit) is limited such that the function of devices positioned downstream—such as air driers, regulating valves—is not adversely affected.
Tests have shown that, in the case in particular of oil-free piston compressors for the rail vehicle industry, the optimum ratio of inner cross section of the bypass pipe to delivery capacity at 10 bar operating pressure is in the range between 0.8 and 1.2. In the event of a compressor arrangement having a plurality of cooler units, said characteristic variable applies only to the post-cooler. The dimensional units for the inner cross section of the bypass pipe is mm2. The dimensional units for the delivery capacity is l/min.
The bypass pipe according to the invention is preferably to be designed, in order to obtain an optimum function, such that in the event of a frozen cooler unit which no longer allows compressed air to pass through, the pressure difference between the two ends of the bypass pipe rises to a maximum of 0.5 bar. A pressure drop of said type is by all means acceptable as a minimal loss of delivery capacity. By the solution according to the invention, in the case of a completely frozen-up cooler unit, the delivery capacity falls by no more than 5% and the outlet temperature downstream of the cooler unit rises by no more than 20° C. in relation to the cooler without a bypass.
According to a further measure which improves the invention, it is provided that the bypass pipe has screw connections at both ends, by means of which screw connections a detachable fastening of the bypass pipe to the cooler unit can take place. By means of said measure, it is also possible to produce cooler units without a bypass pipe as a further product variant. It is also conceivable for the bypass pipe to be integrated entirely into the cooler unit, for example by being soldered or welded in. A pipe composed of steel or a light metal is suitable for the bypass pipe itself, for which purpose it is possible to resort to a standard semi-finished part, for example a pipe with the basic dimensions of 10×1.5 mm. In the selection and dimensioning of the bypass pipe, it is also to be ensured that the latter can withstand the required test pressure.
It is also possible for the bypass pipe to be provided with cooling plates or cooling fins or to be embodied in the manner of a finned pipe in order to improve the cooling action if required. The bypass pipe can be embodied as or can comprise a hollow body produced by casting. In addition to steel, non-metallic materials are also conceivable for production if these are sufficiently temperature-resistant and pressure-resistant. It is for example also conceivable for the bypass pipe to be embodied as or to contain a hose line.
According to another measure which improves the invention, it is provided that the bypass pipe is arranged relative to the cooler unit such that said bypass pipe is situated in the flow of the cooling air which flows through the cooler unit. In this way, it is ensured that the bypass pipe imparts a minimum cooling action to the compressed air flowing through. The bypass pipe can be arranged either vertically or horizontally in relation to the cooler unit and can run straight. If the actual cooling action of the bypass pipe is not sufficient in the case of a straight profile, the bypass pipe can alternatively also be embodied in the manner of a hose pipe or the like.
It is also conceivable for a dewatering/drain valve or a safety valve to be inserted into the wall of the bypass pipe if this is expedient. A dewatering valve would be arranged at the lowest point of the bypass pipe.
The solution according to the invention is suitable for use both with single-stage and also with multi-stage compressor units. In the case of a single-stage compressor unit, the cooler unit is provided as a post-cooler which can then be fitted with at least one bypass pipe according to the invention. In the case of a multi-stage compressor unit, it is conventional for each individual compressor stage to be provided with a cooler unit positioned downstream in the form of an intermediate or post-cooler, with each cooling unit being assigned at least one bypass pipe.
Regardless of the physical embodiment of the compressor unit, the at least one associated bypass pipe is however preferably designed such that, even in the event of the cooler unit being completely frozen up, the entire delivery capacity of the compressor unit positioned upstream can be conducted via the at least one associated bypass pipe.
Further measures which improve the invention are illustrated in more detail below, together with the description of a preferred exemplary embodiment of the invention, on the basis of the figures, in which: